This invention relates to the fabrication of composite materials, and, more particularly, to the fabrication of a molybdenum disilicide-silicon carbide composite material by thermal spray processing.
Molybdenum disilicide (MoSi.sub.2) has great potential as a coating or freestanding structural material for use in elevated-temperature applications at up to about 1600.degree. C. Molybdenum disilicide has a density of about 6.31 grams per cubic centimeter, a melting point of about 2030.degree. C., good oxidation resistance, and high thermal conductivity. However, monolithic molybdenum disilicide exhibits a low fracture toughness at room temperature, low thermal shock resistance, and low creep resistance at elevated temperatures of greater than about 1100.degree. C. These mechanical properties inhibit the utilization of the molybdenum disilicide in otherwise attractive applications.
The mechanical properties of molybdenum disilicide may be significantly improved by forming a composite material of particles of silicon carbide dispersed throughout the molybdenum disilicide. Such composite materials prepared by powder compaction and sintering techniques have exhibited improved room temperature toughness and elevated temperature strength. The presence of the silicon carbide also reduces the coefficient of thermal expansion of the composite material as compared with monolithic molybdenum disilicide. Powder techniques, however, are not practical for many applications, such as certain types of coatings and large freestanding structures.
Other fabrication techniques for composites of molybdenum disilicide and silicon carbide have been proposed. For example, U.S. Pat. No. 5,472,487 discloses the loose mixing of molybdenum disilicide and any of several other types of powders, silicon carbide being one of the disclosed other powders. This loose mixture of separated particles is applied by low pressure plasma spraying of the loose mixture. The present inventor has recognized that this disclosed approach may be well suited for the fabrication of some types of composite materials, but is of limited value in preparing a composite material containing silicon carbide, because of the elevated-temperature sublimation of silicon carbide from the solid state to the gaseous state during the low pressure plasma spraying. The sublimation of the silicon carbide results in its loss from the mixture, so that the amount of silicon carbide in the final product is substantially lower than in the starting material.
There is, accordingly, a need for an improved approach to the fabrication of composite materials containing both molybdenum disilicide and silicon carbide. The present invention fulfills this need, and further provides related advantages.